Healthcare is, and remains, to be one of the most pressing challenges facing the U.S. and the world in the 21st century. A wide range of home-based continuous patient monitoring tools and applications integrated with intelligent and remote decision-making systems are proposed to remedy problems in the widespread delivery of healthcare.
For home-based continuous patient monitoring initiatives, technologies such as digital telecommunications, telemedicine, electronic medical records (EMR), wireless communications, artificial intelligence (AI) and novel medical sensors need to be employed. Although, some of these components such as wireless communications, EMR, and digital communications have been developed at a level that satisfies the requirements of the continuous monitoring applications, some of the key technologies such as the medical sensors still require significant development. Many currently in use medical sensors still require wired data connections, thus, hindering patient mobility, measure only one specific vital sign (VS), and, many are not suitable for continuous monitoring and may be susceptible to motion artifacts and hence not ideal for patient mobility.
There has been no reliable, non-invasive, low-cost, and easy-to-use medical sensor developed to measure a patient's vital signs (VS) as well as other clinically important parameters such as the changes in the lung water content (LWC). The LWC is a medically important parameter since it can be used to reliably detect pulmonary edema at an early stage, and as follow up for treatment in critical burn and heart surgery patients. To overcome these limitations, a microwave stethoscope has been proposed as an integrated, multi-purpose, low-cost, and non-invasive microwave sensor for making multiple VS measurements in addition to LWC from a single microwave measurement, as described by N. Celik, R. Gagarin, H. S. Youn, and M. F. Iskander, “A Non-Invasive microwave sensor and signal processing technique for continuous monitoring of vital signs,” IEEE Antennas and Wireless Propagation Letters, vol. 10, pp. 286-289, February 2011; R. Gagarin, N. Celik, H. S. Youn, and M. F. Iskander, “Microwave Stethoscope: A New Method for Measuring Human Vital Signs,” in 2011 APS-URSI International Conference, Spokane, Wash., July 2011; N. Celik, R. Gagarin, H. S. Youn, J. Baker, and M. F. Iskander, “On the development of a low-cost real-time remote patient monitoring system using a novel non-invasive microwave vital signs sensor,” in IEEE ICWIT Conference, Honolulu, 2010.
The proposed microwave stethoscope was based on microwave reflection coefficient measurements on a patient's chest. The microwave sensor was previously used for LWC measurements using transmission coefficients across the thorax. Studies using animals and isolated lung experiments have validated the feasibility, sensitivity and accuracy of the transmission coefficient measurements in detecting the changes in LWC. It was observed that the measured transmission coefficient includes additional VS data such as heartbeat and respiration. To exploit this additional information, a multi-purpose sensor capable of measuring multiple VS through a single measurement was developed. An integrated system that includes the sensor and a novel digital signal processing (DSP) algorithm was used to extract multiple VS such as respiration rate (RR), respiration amplitude (RA), heart rate (HR), and the heart-beat amplitude (HA) in addition to LWC.
However, microwave measurements based on transmission coefficients have required two properly aligned microwave sensors placed front-to-back across the thorax. High signal attenuation (low SNR) as the signal has to transmit/travel through the entire thorax, reflect and attenuate through many layers of tissue. This made the transmission measurement procedure unusable for large size people, and in some cases an excessive amount of electromagnetic energy (unsafe) was required. Pulsed signal systems were proposed but complicated the systems design and associated DSP algorithms. Maintaining front-to-back sensor alignment also presented problems. In some animal experiments, x-ray images were employed for alignment of the transmission and receiver sensors. The front-to-back transmission approach thus limited the implementation and practical use of microwave measurement technology.
Microwave measurements based on use of a single sensor placed on a patient's chest for transmission and reception of reflection signals were found to provide insufficient signal information. The reflection measurement approach was found to be very insensitive to changes in lung water content and heart related changes vital signs. Reflection signals are dominated by reflection at the surface tissue layers and hence lack sensitivity to desired monitoring of vital signs and changes in lung water content.